Resistive elements can be used as semiconductor switches or memory elements (e.g., memory cells of a memory device), among other applications. Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, resistance variable memory such as phase change random access memory (PCRAM), and resistive random access memory (RRAM), among others.
In modern semiconductor device applications, numerous components are packed onto a single small area, for instance, on a semiconductor substrate, to create an integrated circuit. As the size of integrated circuits is reduced, the components and devices that make up the circuits must be positioned closer together in order to comply with the limited space available. As the industry strives towards a greater density of active components per unit area, effective and accurate creation and isolation between circuit components becomes all the more important.
Memory devices are utilized as non-volatile memory for a wide range of electronic applications in need of high memory densities, high reliability, and low power consumption. Non-volatile memory may be used in a personal computer, a portable memory stick, a solid state drive (SSD), a personal digital assistant (PDA), a digital camera, a cellular telephone, a portable music player (e.g., MP3 player), a movie player, and other electronic devices, among others. Program code and system data, such as a basic input/output system (BIOS), are typically stored in non-volatile memory devices.
Non-volatile resistive memory such as RRAM devices store data by varying the resistance of a resistance element. RRAM devices can have certain beneficial characteristics over other types of memory devices, such as low power consumption, high speed, and excellent bit resolution due to separation and a relatively large resistance ratio between a high resistance state (HRS) and a low resistance state (LRS), without the read/write cycle endurance limitations of charge-storage type memory.
Data may be written to a selected RRAM device by applying a predetermined voltage, at a predetermined polarity, for a predetermined duration. RRAM devices can be operated using two types switching: unipolar or bipolar. Unipolar switching involves programming and erasing using long and short pulses having the same voltage polarity. In contrast, bipolar switching uses short pulses, but programming and erasing pulses are of opposite polarity.
A variety of variable resistance materials have been employed in previous memory cell approaches, including STT-RAM utilizing spin torque characteristics, PCRAM involving the phase change of chalcogenides, Ag ionic transfer technologies, NiO, and copper ionic transport materials. However, many of the technologies of previous approaches do not appear to scale well. Patterning to smaller dimensions is not always possible, and etch damage in forming memory cells becomes a relatively larger problem as memory cell dimensions decrease.
Many previous approaches for implementing memory devices have primarily utilized semiconductor materials for the memory element, reserving the use of metals to that of contacts and conductors. Previous approaches involving the etching of metal can be hindered by poor metal etch rates, use of high processing temperatures, and the use of additional energy sources. These approaches are not practical for semiconductor batch processing of large substrates due to poor etch uniformity, high cost, added equipment complexity, and reliability problems. These, and other, difficulties in using metals so as to achieve smaller feature dimensions have hindered the fabrication efforts of high density RRAM devices.